The present invention relates to an integrated circuit (IC) card and more particularly to a noncontacting IC card.
Since the noncontacting IC card does not contain base electric contacts, it is suitable for a card for controlling a machine tool which splashes a coolant. The IC card is inserted into a reader/writer so that data stored in a random access memory (RAM) in the IC card can be read out or rewritten.
FIG. 5 shows an electronic circuit of a conventional noncontacting IC card. The circuit comprises a control circuit 1 which is an IC having a central processing unit (CPU) and read-only memory (ROM); a random access memory (RAM) 2 which is a memory IC; a power supply circuit 200; and a voltage detector 3. The power supply circuit 200 receives electromagnetic energy from a reader/writer 100 (FIG. 6), for producing power V.sub.DD. The voltage detector 3 detects the voltage of the power V.sub.DD generated from the power supply circuit 200. The electronic circuit of the noncontacting IC card 10 is tightly sealed by an external member of the card so as to be prevented from being affected by atmosphere and dust.
The control circuit 1 is provided with a crystal oscillator 11, capacitors 12, 13 and a CPU. The CPU controls various signals, synchronizing the signals with a clock signal generated by an oscillator provided therein. The RAM 2 has a plurality of memory elements and is applied with access signals, address signals from the control circuit 1 through leads 15 and 16 so as to be activated. Data signals are communicated between the RAM 2 and the control circuit 1 through leads 18.
A backup battery 6 is provided for holding data in the RAM 2 when the IC card is detached from the reader/writer 100. The backup battery 6 is connected to a V.sub.DD terminal of the RAM with a V.sub.DD circuit having inverse current preventing diodes 7 and 8.
As shown in FIG. 2, the voltage detector 3 comprises a comparator 32, a zener diode 33 and resistors. The comparator 32 is supplied with the voltage V.sub.DD from the power supply circuit 200 and produces a signal when the voltage is higher than a predetermined reference voltage. The signal is applied to the control circuit 1 through a lead 31 to release the reset state of the control circuit to start a program.
Referring to FIG. 6, the power supply circuit 200 is provided with coil 201, rectifier diode 202 and smoothing capacitor 203.
In operation, when the IC card 10 is inserted into the reader/writer 100, a coil 101 of the reader/writer 100 generates an alternating magnetic field, and the coil 201 of the IC card receives electromagnetic energy to produce an alternating current. The alternating current is smoothed by the rectifier diode 202. The capacitor 203 is charged with the voltage of the smoothed current and the charged voltage is applied to necessary circuits and devices of the IC card as supply voltage V.sub.DD.
When the voltage at the V.sub.DD terminal of the voltage detector increases from zero to a reference voltage, the control circuit 1 is set so as to start the program.
The control circuit 1 sends a signal to a modulator circuit 301 through a lead 300. The signal is modulated and fed to a transmitter coil 302, thereby applying data stored in the RAM 2 to the reader/writer 100. On the other hand, a signal from the reader/writer 100 is fed to a detector 401 through a receiving coil 402. The signal is detected and applied to the control circuit 1 through a lead 400.
FIG. 3 shows the change of the voltage V.sub.DD when the IC card is inserted into the reader/writer and removed therefrom. The voltage increases to a final maximum voltage E with time.
In general, it takes from several machine cycles to several hundred machine cycles for the supply voltage to reach a sufficient voltage for stabilizing the program operation. Accordingly, while the supply voltage is increased or decreased, the control signal for the RAM 2 is unstable to supply an erroneous control signal to the RAM. This may cause contents of data in the RAM 2 to change.